[Libguestfs] [PATCH virt-v2v] v2v: Allow temporary directory to be set on a global basis.

[Daniel P. Berrangé]

On Thu, Apr 02, 2020 at 05:51:32PM +0200, Pino Toscano wrote:
> On Thursday, 2 April 2020 17:30:39 CEST Richard W.M. Jones wrote:
> > On Thu, Apr 02, 2020 at 03:21:14PM +0200, Pino Toscano wrote:
> > > On Thursday, 2 April 2020 14:49:18 CEST Richard W.M. Jones wrote:
> > > > Previously we placed large files in g#get_cachedir () (usually
> > > > /var/tmp).  However the problem is this ties the libguestfs appliance
> > > > and the virt-v2v overlay files to the same location.
> > > > 
> > > > When virt-v2v is run in a container, or any other situation where
> > > > local storage is limited, it's helpful to be able to put the overlay
> > > > files on an externally mounted PVC, which might be using NFS and
> > > > shared between containers.  But putting the libguestfs appliance on
> > > > NFS in a shared location is certainly not recommended.
> > > > 
> > > > This allows the two locations to be set separately:
> > > > 
> > > >   VIRT_V2V_TMPDIR - location of large temporary files, can use NFS
> > > >                     and may be shared
> > > > 
> > > >   LIBGUESTFS_CACHEDIR - location of libguestfs appliance
> > > > 
> > > > Another motivation for this patch is to allow more reliable cleanup of
> > > > temporary files by an external process, as described in the updated
> > > > documentation.
> > > > ---
> > > 
> > > I do not understand the motivation behind this, which adds yet another
> > > location with temporary files in addition to:
> > > - LIBGUESTFS_TMPDIR - $TMPDIR by default (which itself is /tmp by
> > >   default)
> > > - LIBGUESTFS_CACHEDIR - /var/tmp by default (with a .guestfs-$UID
> > >   subdirectory for the appliance)
> > > 
> > > Before this patch, almost all the temporary files are stored directly
> > > or in subdirectories of $TMPDIR, except big files such as overlays and
> > > OVA extracted content that are in CACHEDIR. With the proposed changes,
> > > _all_ the temporary files will be in CACHEDIR, so there are the
> > > following problems:
> > > - this directory will be cluttered with a lot more files than before
> > > - if it is shared, then other places where it is mounted will see the
> > >   same files
> > > - if it is shared, then creating temporary files will possibly mean
> > >   doing network I/O
> > > - if virt-v2v exits uncleantly, there will be a lot more files to
> > >   cleanup than now
> > > - even without being shared, /var/tmp is persistent unlike /tmp (which
> > >   can be tmpfs-backed on some distros/setups), meaning old temporary
> > >   files will linger way more
> > 
> > How about if we confine the change to just large files (ie. ones
> > which are currently placed in cachedir)?
> 
> This is already the case, isn't it?
> 
> > However the way that virt-v2v works at the moment means you cannot put
> > the large files (especially v2vovl*.qcow2) in a different place from
> > the libguestfs appliance.  This means that if you have only "some"
> > space in /var/tmp -- enough for the appliance, but not enough for the
> > potentially much larger space required by v2vovl*.qcow2 with multiple
> > copies of virt-v2v running -- then you cannot separate the overlays to
> > another directory.  This isn't just a problem for containers.
> 
> /var/tmp is a temporary directory, hence it ought to have enough space
> for big temporary files. This is nothing special for libguestfs or
> virt-v2v.

It is not valid to assume that /var/tmp is arbitrarily big enough
for any files v2v might want to create. AFAIK, there's no real
rule about how large it is expected to be. Linux distros typically
don't do fine grained partitioning by default, so /var/tmp is often
the same filesystem as /, but it is not unusual for admins to have
/ relatively small, and have a separate data partition somewhere
which has the vast majority of storage space available to the host.

In the container world the / is indeed generally of a fairly
limited size, as that's an unpacked container image filesystem
usually on the local host storage.  Apps needing large storage
capacities will usually be given an explicit volume they can
use for this purpose.  NB, the provided volume may be persistent
across container boots, or it may be freshly formatted on every
boot. Whomever configures the container decides this based on
the usage reuqirements of whatever app is to be run in the
container.


> I do not see what makes v2vovl*.qcow2 files so special in this regard,
> requiring them to be handled specially than other big temporary files
> already stored by libguestfs/virt-v2v. Examples:
> - the appliance
> - the temporary directory for qemu when using the libvirt backend
> - the extracted content of an OVA, in case we cannot read it directly
> - the disks when exporting to glance (-o glance)

Out of those things, I think the appliance / QEMU temp dir are in
a different league to the disk images. They're going to be small
enough that I it wouldn't tax the storage capacity of /var/tmp.

Anything related to VM disk images is liable to be an order of
magnitude bigger, easily in the  100's of GB league.

With this POV, I would put  OVA contents, glance exports, and v2v
disk images in the same category, and would expect to be able to
explicitly control  where they are placed, independantly from anything
that goes in /var/tmp.

Regards,
Daniel
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